A series of photoprocesses are used in producing semiconductor devices. Firstly, a mastermask is produced by making reduced copies of reticle patterns, by reducing each pattern to one tenth of its original size. Secondly, a copymask is produced by inversely projecting the pattern of the mastermask. Then, the pattern on the mask is transferred onto a photoresist layer which has been applied on a plain metal layer on a surface of a semiconductor substrate.
A mastermask and a copymask each comprise a transparent substrate, e.g. glass or silica, on which a mask pattern is formed of a metallic film, e.g. chromium, or a metal oxide film, e.g. chromium oxide. The contact exposure technique is commonly used in transferring the pattern from a mask, i.e., light irradiated from the back surface of a mask passes to the plain surface of a photoresist layer to be patterned, where the surface of the photoresist and that of the patterned mask make close contact with each other. However, minute particles of dust of several .mu.m unavoidably adhere to the surface of the transparent substrate of the mask.
During the time of contact exposure, images of the minute particles of dust are transferred together with the mask pattern onto the plain surface to be patterned. Thus, the transferred images of the minute particles of dust result in forming pin holes in the metal conductor when a negative type photoresist is used, and undesired residues of metal are deposited between the pattern designs, when a positive type photoresist is used, leading to disconnection and short circuiting of the metal conductor.